1. Field of the Invention
The present invention relates to a functionally gradient material comprising a metal part composition and a ceramic part composition and prepared in an integrated manner by means of a sintering treatment. The present invention also relates to a method for producing the functionally gradient material.
2. Description of the Related Art
In general, the semiconductor circuit is devised such that the semiconductor circuit is carried on a ceramic substrate to efficiently discharge the heat generated in the semiconductor circuit to the outside in order to stabilize semiconductor characteristics.
In such an arrangement, when the heat is generated from the semiconductor circuit in a relatively large amount, the ceramic substrate cannot sufficiently deal with the heat by itself. For this reason, a heat sink made of copper or aluminum is attached to the ceramic substrate by means of brazing or soldering. In the case of MPU and large capacity electric power IGBT, for example, an artifice is made so that a radiating fin is provided to forcibly release the heat.
The ceramic substrate is required to have high thermal conductivity in order to maintain characteristics of the semiconductor circuit at high levels. Further, the ceramic substrate is required to have insulation property, shielding property, and low dielectric property. On the other hand, the heat sink is also required to have high thermal conductivity. Usually, both of the ceramic substrate and the heat sink are designed to have a thermal conductivity of not less than 150 W/mK and have a coefficient of thermal expansion which is approximate to the coefficient of thermal expansion of the semiconductor chip.
However, the brazing material or the soldering material, which is used to join the ceramic substrate and the heat sink, has a coefficient of thermal expansion which is two-fold or more as compared with those of the ceramic substrate and the heat sink, and a coefficient of thermal conductivity which is not more than 20 W/mK to 70 W/mK. That is, the brazing material or the soldering material has a low value of the coefficient of thermal conductivity which is 1/2 to 1/7 as compared with those of the ceramic substrate and the heat sink. For this reason, the joining section, at which the brazing material or the soldering material is applied, has a low coefficient of thermal expansion. Further, the joining section undergoes large thermal expansion as compared with other sections. A problem is pointed out that a considerably large stress is generated at the joining section, and the reliability of joining is lowered.
Moreover, it is also feared that the heat tends to be accumulated in the joining section, and it is impossible to effectively exhibit the function of the heat sink. Therefore, it is necessary to provide a considerably large heat sink and a considerably large radiating fin so that a large thermal gradient is always maintained. As a result, a problem arises in that such an arrangement cannot respond to the requirement of miniaturization.
Recently, a functionally gradient material has been known, which is integrally provided with characteristics of the ceramic which is excellent, for example, in corrosion resistance, insulation performance, and high temperature durability and the metal which is excellent in toughness. Such a functionally gradient material is usually produced by preparing a laminated compact (including 10 or more layers, if necessary) in which the composition gradually differs, and molding the laminated compact into a predetermined shape, followed by application of a sintering treatment.
However, for example, the sintering temperature, the coefficient of thermal expansion, and the coefficient of thermal conductivity greatly differ between the metal and the ceramic. Therefore, the following problem is pointed out. That is, even if the lamination is made in multiple layers, when the metal and the ceramic are simultaneously sintered, then the ceramic part may remain green or non-sintered, the peeling may occur at the interface between the different compositions, or cracks and breakage may occur. For this reason, the conventional functionally gradient material is not suitable for practical use.
Moreover, in the case of the conventional functionally gradient material, the lamination is made in multiple layers while gradually changing the composition. For this reason, an inconvenience arises in that the moldable thickness is large, it is impossible to obtain any functionally gradient material which has a thin thickness, and hence the conventional functionally gradient material is inferior in performance of wide use. Further, the conventional functionally gradient material involves the following problem. That is, the production steps are complicated, the number of steps is large, and the production cost becomes expensive.
An additional problem is pointed out. That is, when the laminated compact has more multiple layers, the peeling tends to occur during the press molding at the interface at which the composition is changed, and it is impossible to obtain any stable shape. Further, in order to obtain a desired shape, it is necessary to use a large amount of organic additive. Therefore, an inconvenience arises in that the densification during sintering is inhibited, and the metal layer is badly affected.
Further, there is a difference in sintering temperature of 300.degree. C. to 1000.degree. C. between the metal and the ceramic. The densification for the ceramic part does not proceed at a densifying temperature for the metal layer. For this reason, if it is intended to apply heat up to a sintering temperature for the metal layer, the melting points of almost all metals are exceeded. As a result, a problem arises in that the softening takes place, and it is impossible to maintain the shape.
The densifying temperature region for the metal is greatly different from the densifying temperature region for the ceramic, and the coefficient of thermal conductivity and the thermal expansion of the former are greatly different from those of the latter. Therefore, the following problem is pointed out. That is, a large thermal stress occurs to cause warpage and breakage during the sintering, and hence the conventional functionally gradient material is not suitable for practical use.